Track densities continue to increase to meet the demand for storage capacity in the smallest possible space. The demand for increased areal density and ever larger disk storage capacities requires a lower flying height (or lower Head Media Spacing or HMS) of the read/write head above the disk surface. Lower clearances equate to higher track densities and higher bit rates.
Disk surface morphology can have a substantial impact on read/write head performance and therefore areal density. Substantial fluctuations in surface curvature, whether circumferential or tangential curvature or radial curvature, can cause fly height modulation as the head moves over the disk surface. Magnetic flux density in the air gap changes by the cube of the distance. Fly height modulation during the write cycle can decrease the flux density to a level where the flux density is not sufficient to saturate the media. Moreover, fly height modulation during the read cycle can decrease the flux density to a level where data cannot be read accurately. As a consequence, the flatness, waviness, and roughness features on the disk surface may be the next practical barrier to increasing storage capacity.
Current disk manufacturing processes attempt to manufacture disks having only small fluctuations in surface and radial curvatures. In a typical manufacturing process, the disk is ground, polished, washed, rinsed, and tape burnished and wiped. In tape burnishing and wiping, the disk is polished while it spins around a spindle. After buffing and burnishing, the substrate is wiped and a clean lube is evenly applied on the surface. The lube is commonly applied by a dip lubrication process in which the disks are dipped or immersed in a bath of the lubricant. The disks may then be removed from and suspended above the bath to permit the excess lubricant to drip into the bath and the lubricant to dry. Alternatively, the bath vessel may be drained of the lubricant while the positions of the disks are maintained in the vessel. In either event, the disk is removed slowly from the lubricant bath so that, under the force of gravity, only a thin film of lubricant forms on the disk surface.
The disk is prepared and tested for quality through a three-stage process. First, a burnishing head passes over the surface, removing any bumps (known as asperites) and increasing the bond strength between the lubricant layer and underlying protective layer. The glide head then passes over the disk, checking for any remaining asperites. Finally, the certifying head checks the surface for manufacturing defects and measures the magnetic recording ability of the substrate.
Nothwithstanding the foregoing process, there have still been problems with flying height modulations.